The production of sintered parts from "green" bodies is well known in the art. Generally, the green body is formed by filling a die with a powder/binder mixture and compacting the mixture under pressure to produce the green body. The green body, a self supporting structure, is then removed from the die and sintered. During the sintering process, the binder is volatilized and burned out. However, removal of the binder can cause the product to crack, shrink and/or become distorted.
The injection molding of metal parts from powders has been a particularly troublesome process, and notably, processes based on water as the fluid transporting medium. It is well known that finely divided metal powders (M) can react with water (H.sub.2 O) to form oxides on the surface according to: EQU xM+yH.sub.2 O=MxOy+yH.sub.2
(Metals Handbook, vol 7, p36, American Society for Metals, Metals Park, Ohio, 1984). It is also recognized that the thickness of the oxide film is inversely proportional to the particle size of the metal powder (Metals Handbook, vol 7, p37, American Society for Metals, Metals Park, Ohio, 1984). Impurities, in particular, surface oxides, can lead to weak interparticle bonding during sintering, resulting in inferior mechanical properties in the fired part (R.M. German, Powder Metallurgy Science, p304, Metal Powder Industries Federation, Princeton, N.J., 1994).
Recently, a water-based process using methylcellulose polymers as binders in the manufacture of parts from metal powders has been disclosed. U.S. Pat. No. 4,113,480 discloses the use of methylcellulose or other plastic media (e.g., polyvinyl alcohol) and water in forming injection molded metallic parts. With respect to the mechanical properties of the final parts, however, the elongation disclosed in the tabulated mechanical properties is only marginal at 2.6% and 2.5%. Moreover, aqueous solutions of methylcellulose are fluid at temperatures around 25.degree. C. and gel at elevated temperatures roughly in the range 50-100.degree. C. This particular mode of gelling behavior necessitates molding from a cool barrel into a heated die. The elevated die temperature can cause the molded part to lose water by evaporation prematurely before it is totally formed, resulting in non-uniform density in the molded part. This density inhomogeneity can lead to cracking and warping in subsequent processing steps of drying and sintering.
The use of aqueous solutions of agaroids as binders for injection molding ceramic and metal parts is disclosed in U.S. Pat No. 4,734,237. However, examples containing only ceramic compositions are given and no stainless steel compositions are provided.
Suitable injection molding compositions must be those which are capable of transforming from a highly fluid state (necessary for the injection step to proceed) to a solid state having a high green strength (necessary for subsequent handling).
In order to meet these requirements, and avoid the potentially damaging metal-water chemical reactions, most prior art molding compositions comprise a relatively high percentage of a low melting point binder, such as wax (R. M. German, Powder Injection Molding, Metal Powder Industries Federation, Princeton, N.J., 1990). However, such systems exhibit a number of problems in forming parts, especially parts of complex shapes.
More specifically, waxes are commonly employed as binders because they exhibit desirable rheological properties such as high fluidity at moderately elevated temperatures and substantial rigidity at temperature below about 25.degree. C. Wax formulations normally comprise between about 35% and about 45% organic binder by volume of the formula. During the firing process, wax is initially removed from the body in liquid form. During this initial step of the firing process, the green body may disintegrate or become distorted. Consequently, it is often necessary to preserve the shape of the green body by immersing it in an absorbent refractory powder (capable of absorbing the liquid wax). Notwithstanding the use of the supporting powder to retain the shape of the body, the formation of complex shapes from wax-based systems is even more difficult because it requires, in most instances, detailed firing schedules which may encompass several days in an attempt to avoid the development of cracks in the part.
In spite of the problems associated with aqueous compositions of metallic powders enumerated above we have found, unexpectedly, novel molding compositions useful in forming complex shapes which can be fired to stainless steel products with excellent mechanical properties. Furthermore, the novel molding compositions disclosed are useful in forming stainless steel parts which not only reduce the firing times and regimens for such parts, but also allow for the production of complex shapes without the attendant shrinkage and cracking problems associated with the prior art products. Moreover, the compositions can be molded in the "conventional" manner, i.e., from a heated injection molding barrel into a cool die.
Generally speaking stainless steels refer to Fe/Cr alloys. Invariably, other elements are included to attain certain properties. Five classes of stainless are delineated in the metals handbook (Metals Handbook, Tenth Edition, Vol 1, ASM International, Materials Park, Ohio, 1990) comprising austenitic, ferritic, martensitic, duplex and precipitation hardened alloys. B asic formulations for the five categories are given on p 843 of the h a ndbook. The elements frequently alloyed with Fe and Cr comprise Ni, Mn, Mo, Al, Nb, Ti, Ca, Co, Cu, V, and W.